The water resources of the Marahau catchment, Tasman, New Zealand

Abstract

The Marahau River catchment is located on the western side of Tasman Bay, at the northern end of New Zealand's South Island. Very little existing hydrological data are available from the catchment and this study included a program of data collection. The long-term mean totals for the collected data were estimated by comparing this data with nearby existing data. Mean river flow was estimated at 1.30 m³/s, evaporative losses 760 mm/a, and rainfall was found to range from 2200 mm/a at the coast to 3500 mm/a at higher altitudes. Although receiving moderate rainfall, the area receives a winter maximum and summers are typically characterised by long dry periods.
An unconfined aquifer (the Marahau Unconfined Aquifer) is found within the coastal flats. The aquifer is of moderate permeability (300 - 400 m²/day) and consists of accumulated coarse granitic sand and granitic river cobbles. Recharge to the aquifer is from rainfall infiltration and leakage from several small streams that flow across the aquifer. The lower Marahau River itself is not a source of recharge to the aquifer. Groundwater is typically acidic with moderate iron concentrations.
The settlement of Marahau, located at the coast, is the southern gateway to the Abel Tasman National Park. Increasing visitor numbers to the Park and associated commercial activity at Marahau has led to increased growth of the community and an increase in demand for Marahau's water resources. Presently water is obtained either individually or collectively from several sources including: groundwater, surface water, natural springs and seeps, and rainfall collection. Of these only groundwater and surface water provide an adequate supply in terms of both quantity and quality for present demands.
Groundwater is increasingly being used as a water supply with its use and development to date being in an ad hoc manner. Current groundwater abstraction is estimated at 50,000 l/day (0.58 l/s) and expected to be well within the sustainable yield of the aquifer. Parts of the aquifer are vulnerable and abstraction of relatively small volumes of water in some areas may result in adverse localised groundwater flow directions. Such flow directions could result in contamination of wells with saline water from the coast or discharge from septic tanks. To date there have been no reported instances of this occurring. There is sufficient groundwater at Marahau for the foreseeable demand (including the possibility of a future reticulated water supply) provided use of the resource is well managed and monitored.... [Show full abstract]